In anhydrous pyridine solution at 294 K, U(III) and Ce(III) triiodides were found to form both 1: 1 (ML) and 1:2 (ML2) complexes with bipyridine (bipy = L) while Nd(III) triodide formed only a 1:2 complex. The 1:3 (ML3) complexes were identified at low temperature with a large excess of L. Conductometry measurements showed for U(III) a large increase in the conductivity when increasing the molar ratio L:U. The complex UL2 was found to be a 1: 1 electrolyte and the species UI2+ was more reactive toward L in comparison with UI3. For Ce(IH) and Nd(IH), MI2+ and MI3 present about the same affinity for L. The stability of the complexes is limited, and U(III) possesses a slightly higher affinity for bipy than the trivalent lanthanides. Interestingly, a preference for the formation of ML2 Complex was shown for all the studied M(III) ions. The driving force for complex formation was always the enthalpy, and, surprisingly for a bidendate ligand (bipy), no favorable entropy contribution to complex formation was observed. The X-ray crystal structures of [CeI3(bipy)(2)(py)](4)(.)5py-bipy and UI3(bipy)(2)(py)(.)2py were determined. The structures of the molecules MI3(biPY)(2)(Py) are almost identical for U and Ce. The mean M(III)-N(bipy) bond distances are equal to 2.67(3) Angstrom for Ce(IH) and 2.65(4) Angstrom for U(III). The slightly smaller M(IH)-N(bipy) distances observed for U(III) would reflect a slightly more important covalent character of the U(III)-N(bipy) bonds, in agreement with the slightly better affinity of U(III) than Ce(III) or Nd(HI) toward bipy observed in solution and with the fact, that the enthalpy is the driving force for complex formation.